How much should it cost to replace the spark plugs on 2002 Cadillac DTS?

I was quoted $400 from Germain Cadillac. They told me this is the reason my car is idling rough and vibrating at high speeds, oh and the biggie, wasting fuel! I just wanted to see $400 is reasonable or am I paying that since its a Cadillac dealership?|||Spark plugs are as cheap as $2 each but even if you pay 5 dollars for each that's $30-40...your getting ripped off|||This sounds about average to me.|||If you think it's too much, take it elsewhere or do it yourself. It's more than I would pay.|||I would put plugs in it twice for that price|||$375-$425